Application Research on Air-Bearing Centrifugal Blowers for Energy Saving and Noise Reduction in the Cement Industry
Wang Xinming (Weifang FuYuan Turbocharger Co., Ltd., Weifang, Shandong 261205)
Abstract: The air-bearing centrifugal blower is a new type of efficient, energy-saving and environmentally friendly centrifugal blower. This paper describes its construction and operating principle. Compared with conventional blowers it offers energy saving, low noise, oil-free operation, reliability, simple operation and a long service life; it needs no auxiliary systems, and its footprint and maintenance workload are small. An actual energy-saving blower retrofit at a cement plant shows that when air-bearing centrifugal blowers replace Roots blowers for pulverized-coal conveying and air supply to the rotary kiln, for raw meal silos and for other stages of cement production, the energy saving is significant and noise falls markedly.
Keywords: air-bearing centrifugal blower, cement, material conveying, gas conveying, power consumption, noise.
Introduction:
Blowers play an important part in cement production. Against the current background of energy saving and emission reduction, the shift from old to new growth drivers and routine staggered-peak production, the environmental requirements facing the cement industry are becoming ever more stringent, and saving energy and reducing consumption is imperative. Blowers and fans consume a great deal of energy in cement production, about 30%–35% of the total. Reducing blower energy consumption is therefore key to saving energy in cement production. Conventional blowers have reached their limits in this respect. The air-bearing centrifugal blower offers a way forward: it dispenses with the speed-increasing gearbox, coupling, lubrication system and cooling fan that conventional blowers require, and uses a high-speed permanent-magnet synchronous motor, aerodynamic foil bearings, a three-dimensional-flow centrifugal impeller and variable-frequency control. Its speed can be adjusted to the operating requirement so that the machine always runs efficiently, which effectively lowers energy consumption, avoids secondary pollution of the environment by noise, vibration and waste lubricating oil, reduces the maintenance workload and saves maintenance costs. This is of real practical importance for the high-quality development of the cement industry and for the competitiveness of cement producers.
1 The air-bearing centrifugal blower
1.1 Principle
The air-bearing centrifugal blower is a high-speed turbomachine of integrated, compact, skid-mounted design. Its core technologies are aerodynamic foil bearings, a high-speed permanent-magnet synchronous motor, a three-dimensional-flow centrifugal impeller, variable-frequency control, and measurement and monitoring. The basic principle is that a high-speed permanent-magnet synchronous motor, supported on air bearings and controlled by a variable-frequency drive, directly drives a centrifugal impeller on the same shaft. Air is drawn in through the inlet of the volute, is compressed by the centrifugal action of the impeller to a given velocity and pressure, and leaves through the volute outlet. The continuous rotation of the impeller draws in and discharges air without interruption, giving a continuous air supply. High-performance variable-frequency control makes the blower's control strategy simpler and its running smoother, and widens its adjustable operating range.
1.2 Construction
(1) Aerodynamic foil bearing
The aerodynamic foil bearing is a self-acting air bearing in which the shaft is supported by compliant foils. Before the blower starts, the rotor journal is in physical contact with the bearing. On start-up the rotor moves relative to the bearing; the rotating rotor entrains air and generates gas pressure that lifts the bearing foils away from the rotor, and a complete air film forms between the foils and the rotor by aerodynamic action. The bearing thus suspends the rotor on a field of air pressure. When the rotor runs at high speed there is no physical contact between shaft and bearing, no lubricating oil is needed and energy losses are extremely low. The foil surfaces have a high-temperature solid coating and need no maintenance. The bearing can complete more than 20000 trouble-free start-stop cycles; at one start-stop a day that corresponds to 54.8 years of operation, so the bearing is semi-permanent.
(2) High-speed permanent-magnet synchronous motor
The motor uses world-class permanent-magnet synchronous motor technology and is driven and speed-controlled by a high-performance variable-frequency drive. Motor speed can reach 150000 rpm and efficiency exceeds 97%. The motor rotor is connected directly to the impeller, so there are no transmission losses and transmission efficiency can reach 100%.
(3) Three-dimensional-flow centrifugal impeller
The impeller is of the semi-open type, designed with three-dimensional flow theory and parametrically optimized, giving high efficiency and a wide flow range. It is made of high-strength forged aluminum, machined to shape on a five-axis machining center, and is highly resistant to deformation. Peak impeller efficiency at the operating point can reach 85%, and the service life is more than 30 years.
(4) Variable-frequency control and measurement and monitoring
The blower is controlled by a PLC and a sensorless-vector variable-frequency drive. The speed of the permanent-magnet motor is controlled by adjusting the frequency, which in turn controls the blower's flow and discharge pressure to suit all operating conditions. The intelligent control system can predict surge and provides local control, and the installed measurement and monitoring system allows remote monitoring and data transmission.
1.3 Main performance advantages
(1) High efficiency and energy saving
The air-bearing centrifugal blower uses advanced technologies including a high-efficiency centrifugal impeller, direct drive by a high-speed permanent-magnet synchronous motor and non-contact aerodynamic foil bearings. In operation it adjusts the motor speed automatically to the air flow required, which keeps the machine running at high efficiency. It can save 20%–30% of energy compared with a conventional Roots blower, 15%–20% compared with a conventional multistage centrifugal blower and 10%–15% compared with a geared single-stage centrifugal blower.
(2) No vibration, low noise
With aerodynamic bearings there is no contact between the rotating parts and the mechanical system and no mechanical friction, so the blower runs smoothly and without vibration. The noise of the complete unit at 1 m is below 80 dB, and no sound insulation is needed.
(3) No lubricating oil, no mechanical servicing, easy maintenance
Aerodynamic bearings and direct drive eliminate the complex gearbox and oil-lubricated bearing system that conventional blowers require. The system needs no lubrication system, no lubricating oil at all and no mechanical servicing; routine maintenance consists only of replacing the air filter, which effectively lowers the user's maintenance costs.
(4) High level of system integration
Local control and the variable-frequency drive are integrated in one unit. All functions can be set from the control panel, where speed, temperature, pressure, flow, power consumption and other data can also be viewed. A high-efficiency filter, cooling system, anti-surge system, protection modes for unexpected power failure and faults, and a real-time touchscreen make operation more convenient and reduce operating errors.
(5) Easy installation
The complete unit is of integrated, compact, skid-mounted design; it is small and simple to install and operate.